Ranked best to worst:
DB is my favorite, I just love being hired to kill people (and awsome quests such as whodunit?)
Theives guild a close 2nd, I love stealing stuff, and it can get you quite alot of money stealing stuff and selling to fences
Arena would probley be 3rd, I just like fighting, all there is too it O.O
Mages guild next, would be ranked 2nd if they didnt have those stupid guild recomendations in the beginning :facepalm:
Fighters guild would be last, while some of the quests are fun, some of them are just dull and boring :shrug: